Metal: Fix compressed texture initialization and add DXT1 test There was a divergence between WebKit and upstream ANGLE in InitializeTextureContents' handling of compressed textures. Align upstream ANGLE with WebKit, fix a problem with emulated compressed texture formats like ETC2, and add a new test: DXT1CompressedTextureTestWebGL2.InitializeTextureContents Bug: chromium:1289428 Change-Id: Ieac6b99a4b19a34d0d87b7577882760ea395c605 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3406746 Reviewed-by: Kyle Piddington <kpiddington@apple.com> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>